EE9800002

10 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE

Kalju Eerme Tartu Observatory, Toomemdgi, EE2400 Tartu

Introduction For natural reasons, the global climate changes continually in different time scales as related to the processes dominant. The slowest changes, in millions and tens of millions of years, are caused by continental drift and global tectonics. The processes initiated by periodic changes in the Earth's orbital parameters occur in the time scale of tens and hundreds of thousands of years. The fastest changes are related to the changes in atmospheric composition as well as in atmospheric circulation and oceanic currents. A specific feature of present-day changes is a significant anthropogenic impact on atmospheric trace gases. Such an impact can be regarded as a test on natural climate feedbacks. It is believed that never before has the composition of the atmospheric trace gases changed so rapidly as at present.

Basically, global climate is determined by the global energy budget and redistribution of accumulated energy. This redistribution is realized by oceanic currents and atmospheric winds. Both energy supply and redistribution efficiency depend on the wide variety of processes taking place in the atmosphere, hydrosphere, biosphere, cryosphere, and soils. In short time scales, in tens of years, the changes in the atmospheric concentration of certain trace gases lead to the most significant climate forcing. The dynamic processes in the atmosphere and in the ocean are sensitive to small temperature deviations. Systematic increases or decreases in temperature are always accompanied by shifts in weather patterns and currents. As greenhouse gases (GHG) have controlled the climates in the past, so they are controlling the present climate. In a more detailed analysis, the GHG can be divided into radiatively active and chemically active gases. Some of them, for example, are simultaneously both radiatively and chemically active. Radiatively active gases are immediatly interactive with the radiation penetrating the atmosphere, while chemically active gases control the atmospheric content of radiatively active gases. Typically, the atmospheric GHG have sources and sinks outside the atmosphere. If the corresponding fluxes are imbalanced for some reason, then the atmospheric pool can increase or decrease. At present, the atmospheric pools of several major GHG are increasing. This increase contains significant irregularities, which are still poorly understood (Khalil & Rasmussen, 1994; Siegenthaler & Sarmiento, 1993; IPCC, 1994).

The problem of GHG and is closely linked to the problem of atmospheric ozone. A certain part of GHG acts in the chemistry of the ground level ozone and in the stratospheric . A troubling feature is that part of the gases causing the global climate change and stratospheric ozone depletion are man-made. For this reason, the current atmospheric chlorine and bromine burden is much higher than it could ever be in natural conditions.In the case of such perturbed chemical composition, the atmospheric cleansing processes are inhibited. Obviously, it 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE 11 restricts the application of the past climate analogy for estimating the present-day and future climate changes.

The GHG budgets For modeling present and future climate changes, complete data about the GHG is crucial. It must contain the intensities of sources and sinks, the rates of atmospheric increase, the participation in chemical reactions and the transport tracks inside the atmosphere.

The key species among the approximately 40 different GHG are CO2, CH4, NO2 and the tropospheric O3. On a global scale, water vapor is responsible for about 70% of the total greenhouse warming (IPCC, 1990). Usually, water vapor is not included in the listing of GHG gases. It is believed not to have global anthropogenic impact as a short-lived gas in the troposphere. Its mean tropospheric lifetime is about 10 days, similar to the mean lifetime of atmospheric boundary layer aerosol. Both tropospheric water and aerosol are removed by precipitation. In this case, being globally integrated, atmospheric water vapor does not cause any increase of global warming at present, but its local effects can be remarkable. The atmospheric distribution of water vapor is essentially nonuniform. There are certain preferable pathways of water vapor movement sometimes called the "tropospheric rivers" (Newell et al., 1992). Climate change certainly changes the "river beds" and can amplify the regional greenhouse warming. There are significant diurnal and seasonal fluctuations in the total column water vapour content. At least over the continental Eurasian area, a linear trend of increase in the column water vapor content has been observed during the last decade (Arefev et al., 1995). Obviously, there is a necessity for more detailed studies of the column water vapor content in different geographical regions. If such an increase is common, then additional warming due to water vapor cannot be neglected. At the present time, it is possible to use the Global Positional System for the column water vapor content observations. A detailed treatment of water vapor content change is essential in the regions of changing air mass movement.

The problem of how to calculate the global budgets of major GHG has been discussed from different viewpoints. The most logical way seems to be calculating the budgets separately for each country, as proposed for the Country Study Program. If local budgets are calculated with a sufficiently high accuracy, then the global budgets will undoubtedly be better specified. On the other hand, these more exact local balances have no significant benefit for the prediction of local climate.

GHG, with the exception of water vapor and quickly oxidizing species, such as CO and some hydrocarbons, have long atmospheric lifetimes. For this reason, they will be well mixed in the atmosphere, and the global pattern of their sources and sinks is of less importance than the total intensities. Among the sources, the best known are industrial emissions, which are estimated with an

accuracy of about ± 10% (King et al., 1995; Marland et al.,1989) for the -containing CO2 and CH4 gases. The estimates of natural emissions and consumptions have a much lower accuracy. On a global scale, both the land use changes and gas exchange rates still have significant uncertainty. For these reasons, the global net exchange rates have an uncertainty of the order ± 60% (IPCC, 1992). It is noteworthy that the elemental constituents of GHG are mostly the macroelements of biological tissues. The natural exchange between biosphere and atmosphere is thus regulated by the intensity of biological processes. There is an urgent need to understand better the extent of coupling between the plant growth, global temperature and enhanced GHG concentration. The gas exchange, in particular, is realized by the microbial decay processes which are regulated by similar feedbacks. In the case of sulphur-containing gases, the climatic feedback is realized by the aerosol production in the 12 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE atmosphere. It is believed that the main climate feedback of marine algae is realized by their emission of dimetyl sulphide after transformation into the sulphate aerosol. The problem of dimetylsulphide emissions is quite complicated. A good survey can be found in Lawrence (1993). The release of dimetylsulphide into the surrounding water occurs only in the case of certain species of marine phytoplankton. The release depends not as much on the phytoplankton concentration as on the zooplankton grazing. In regions where the zooplankton population is high relative to the phytoplankton population, the highest dimetylsulphide releases can occur. The dimetylsulphide is released into the water primarily via lysis of phytoplankton cell walls. Thus, the production rates depend on the death rates of phytoplankton of the certain dimetylsulphide producing classes. The next steps leading from the dissolved dimetylsulphide in the water to the particular sulphate in the air are dependent on the gas exchange between the water and the air, and on the oxidation and particle formation rates. The main climatic influence of land plants is realized by the fixation of atmospheric (Lovelock & Kump, 1994). For example, the carbon dioxide uptake is regulated by the El Nino Southern Oscillation. During the El Nino Warming events, the uptake by the terrestrial vegetation is reduced, resulting from the collapse of the Southeast Asian monsoon (Sarmiento, 1993). The long-lived GHG can be well mixed in the atmosphere, and their forcing is only latitude dependent due to different illumination conditions. The climate forcing of aerosols has a strong regional character. The anthropogenic emissions of sulphur dioxide from industry and power plants produce a local increase in sulphate aerosol abundances. The direct effect of local aerosol negative forcing is accompanied by the production of cloud condensation nuclei, supplementing the indirect forcing caused by the higher reflectance of clouds (Taylor & Penner, 1994). In some cases, specialized studies of the regional climate forcing by the increased aerosol production may be necessary. Most of Europe belongs to the region of high aerosol production.

In developed countries, the estimations of GHG budgets are supported by studies on the net flux measurements and the absorption-emission process levels. The process intensities and correspondingly, the net fluxes are sensitive to the background environmental changes, which are not strongly predictable. It is a key question how living organisms participate in climate regulation and how a coupled system including the biota, the atmosphere, the ocean, and the rocks operates. What effects do the organisms have on the environmental conditions and what effects do changed environmental conditions have on the organisms' growth and response? Currently, the flux measurements are made by the closed chamber or the enclosure method, and the eddy correlation method. The chamber method is significantly simpler and cheaper. Chambers of different complexity of construction and of different cost are in use (Roulet et al., 1992; Klinger et al., 1994; Valente & Thornton, 1993). The gas chromatograhy of the samples is the main method used in these studies.lt is difficult to obtain data, which is representative of an extended area using the chamber method by itself. Because of its high spatial resolution, this method is useful in exchange process studies in soils and plant canopies. The eddy correlation method has been elaborated for ecosystem- atmosphere exchange studies. This method combines the precise micrometeorological wind speed components and the gas flux measurements. Such a method has been widely used for the evaluation of vertical fluxes of heat, momentum and water vapor in natural environments for a long time. It is assumed that the vertical fluxes are constant in the atmospheric boundary layer. For climate change- oriented studies, the eddy correlation method is more representative than the enclosure method. Unfortunately, in Estonia's present conditions, this instrumentation seems to be too expensive. The technique of tunable diode laser spectroscopy is applied for the detection of atmospheric trace gas fluxes. Sonic anemometry is applied to detect fluctuations in the wind velocity components. The covariance of the vertical wind velocity and gas concentration, calculated over a time period, will 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE 13_ represent the flux of the gas. Abandonment of the eddy correlation method means that the GHG budgets cannot be detected with sufficient accuracy, at least as they relate to the exchange between the soil and plant canopies and the atmosphere.

The problem of reducing the GHG emissions in Estonia's present conditions is fortunately not typical of the world, but much easier. Since 1990, industrial emissions of GHG and atmospheric pollutants have been reduced more than a third due to reduced industrial production. Changes in land use are directed towards an increase of forested area. Taking into account quite a wide variety of natural conditions, there is a very good possibility of specialized studies. The most interesting research area seems to be how the GHG absorption-emission in soils and plant canopies depends on seasonal warming and other climate-related changes. Which are the governing processes, and how are they related to the gas exchange? Similar studies have been widely spread in developed countries during recent years (for example TIGER Eye No. 13, 14 , 1995).

The GHG and the ozone Frequently, the greenhouse warming and the stratospheric ozone depletion are treated separately. But a significant number of gases responsible for greenhouse warming coincide with those responsible for ozone depletion. Those gases are characterized by a (GWP) on the one hand and by an ozone depleting potential (ODP) on the other. Even gases having no direct influence on ozone, such as CO2, enhance ozone depletion through stratospheric cooling. It is necessary to underline that cooling in the stratosphere and mesosphere due to the increased GHG content is much more evident than tropospheric warming. The total number of compensating feedbacks in these atmospheric layers is lower than those for the troposphere. The melting and freezing of water connected with significant heat release and absorption and direct biospheric effects are unique to the troposphere. The GWP are dependent on the integrated absorption cross sections in the spectral region of the atmospheric infrared window and on the atmospheric lifetimes of gases. The reference gas is carbon dioxide. For other gases, the GWP is determined on a per molecule basis. The GHG have different atmospheric lifetimes, and correspondingly, the values of GWP depend on the time period of integration. For example, in the case of integration over a 100 year period, the GWP of the methane is 24.5 times higher and the GWP of CF2C12 or CFC-12 8500 times higher than GWP of CO2 (IPCC, 1994). The recently revised values are significantly higher than those published a few years ago. In 1991 (WMO, 1991), the corresponding GWP values were 11 and 7 100.

The climatic influence of halocarbons is realized in two different ways. Their current direct radiative warming is well quantified. But their net forcing contains an additional cooling due to stratospheric ozone destruction. The magnitude of this cooling depends strongly on the altitude of the ozone loss. At present, the halocarbon-induced direct heating is nearly compensated by indirect cooling (Daniel et al., 1995). This balance seems to be destroyed in the future. If the compounds destroying ozone are replaced by the "ozone-friendly" ones, then their greenhouse heating will not be compensated by indirect cooling.

The ODP represents the amount of ozone destroyed by emission of a gas over its entire atmospheric lifetime relative to that due to emission of the same mass of CFC-11 (or CFCI3). The ODP is a 14 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE relative measure of ozone depletion in the same way that the GWP is a relative measure of greenhouse warming. The chlorine and bromine compounds having significant ODP are often characterized with regard to the chlorine loading potential (CLP) or the bromine loading potential (BLP). The chemical depletion of stratospheric ozone depends most significantly on total chlorine and bromine burden.

The influence of depleted stratospheric ozone on GHG balance is realized through short wavelength irradiance. The increase of UV-B irradiance affects the terrestrial and oceanic ecosystems and the gas exchange between them and the atmosphere. At present, these processes are poorly understood. Another ozone problem is the ground level ozone and the atmospheric oxidizing capacity problem.The atmospheric balances of some greenhouse gases can be disturbed by the changes in atmospheric oxidizing capacity. A potential danger to methane oxidizing capacity of the lower atmosphere is an increasing use of the "ozone friendly" substances instead of . A certain part of the atmospheric OH radicals can be used for oxidizing these hydrochlorofluorocarbons, and changes in oxidizing capacity of CO, methane and some other greenhouse gases can occur (Fuglestvedt et al., 1994). Now it is believed that the most significant disturbances by tropospheric ozone and UV-B radiation lead to changes of microbial processes in soils and plants.

Carbon dioxide stands in a key position, not only as a major GHG, but as a carbon source for plant growth. Some recent studies (Rotmans & den Elzen, 1993; Friend et al., 1993 ) have shown that the elevated atmospheric concentration of COi induces significant changes in plant physiology. The acclimation to elevated CO2 levels has become a central problem. Some plant species react to higher CO2 concentrations by reducing their stomatal aperture (Pearson, 1995). At the same time, their water loss becomes reduced, and its redistribution between different species in the canopy occurs. This combination of modified gas exchange and water use changes the photosynthetic rates of the species and CO2 balance of the whole canopy. Most of the currently-used climate models are linear and do not take into account the response changes. Due to this, extrapolations may present significantly unrealistic results. Still, the response changes are poorly studied, and it is virtually impossible to take them into account. Estonian landscapes containing a wide variety of different soils and growth conditions can be considered a perspective area for response studies.

Change in Estonian climate The climate in a certain geographical location is dominantly influenced by atmospheric air masses and their movement. Estonia is geographically located on the crossing of different "imported" air masses. The impact of the local balance of GHG and the local land surface properties on the climatic air mass is small. The prediction of future Estonian climates will require the prediction of trajectories of the climate-forming air masses. Obviously, the origin of the dominating air masses is seasonally different, as are the speed and direction of their movement. To model and predict climate change is a very complicated endeavor, largely because future predictions cannot be checked immediately. The comparison of different models and climatic scenarios has been made on the basis of known past climates. Often, future climates have been predicted on the basis of analogous past climates or on that of very superficial general circulation models. Contemporary atmospheric chemistry and relative composition of trace gases are not directly comparable with past ones. The spatial smoothing of 2. GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE 15_ general circulation models facilitates work at the level of climate zones, but not at the level of such small spatial details as Estonia represents.

For the most part, local climate changes are generated by changes in general circulation. Global warming does not have a direct impact on global climate, but acts through the cyclogenetic and other dynamical processes. In Estonian conditions, winter is believed to be the most sensitive season to climate change. The generation and movement of the determining climate air masses depend on the location of the polar front and on cyclogenetic activity over the Northern Atlantic. Specific effects include regional forcing by the water vapor, clouds and aerosol. In summer and fall, the climate- forming air can have a different origin and can be more stable. Still, it is poorly understood how the local effects of atmospheric circulation depend on the global energy distribution pattern and its changes.

References Arefev, V.N., Kamenogradsky, N.Ye., Kashin, F.V. & Ustinov.V.P. (1995). Investigation of total column water vapor content in the atmosphere. Izv. AN. Fizika atmosfery i okeana, 31, pp. 660-666 (in Russian).

Daniel J.S., Solomon, S. & Albritton, D.L. (1995). On the evaluation of halocarbon and Global Warming Potentials. J. Geophys. Res., 100, Dl, pp. 1271-1285.

Friend, A.D., Shugart, H.H. & Running, S.W. (1993). A physiology-based Gap Model of forest dynamics. Ecology, 74, pp. 792-797'.

Fuglestvedt, J.S., Jonson, J.E. & IsaksenJ.S.A. (1994). Effects of reductions of stratospheric ozone on tropospheric chemistry through changes in photolysis rates. Tellus,. 46B, pp. 172-192.

IPCC (1990). Houghton, J.T., Jenkins, G.J. & Ephraums, J.J. (eds.). Climate Change:The IPCC Scientific Assessment. Intergovernmental Panel on Climate Change. Cambridge University Press.

IPCC (1992). Houghton, J.T., Callander,B.A. & Varney,S.K. (eds.). Climate Change: the Supplementary Report to the IPCC Scientific Assessment. Intergovernmental Panel on Climate Change. Cambridge University Press.

IPCC (1994). Radiative Forcing of Climate Change:The 1994 Report of the Scientific Assessment Working Group of IPCC. Summary for Policymakers. WMO. UNEP.

Khalil, M.A.K. & Rasmussen, R.A. (1994). Global decrease in atmospheric carbon monoxide concentration. Nature, 370, pp.639-641.

King, A.W., Emanuel, W.R. Wullschleger, S.D. & Post, W.M. (1995). In search of the missing : a model of terrestrial biospheric response to land-use change and atmospheric CO2.. Tellus, 47B, pp. 501-519. 16 2, GREENHOUSE GASES AND THEIR IMPACT ON THE RADIATION BALANCE

Klinger, L.F., Zimmermann, P.R., Greenberg, J.P., Heidt, L.E. & Guenther, A.B. (1994). Carbon trace gas fluxes along a successional gradient in the Hudson Bay Lowland. J. Geophys. Res., 99, Dl, pp. 1469-1494.

Lawrence, M.G. (1993). An empirical analysis of the strength of the phytoplankton-dimetylsulfide- cloud-climate feedback cycle. J. Geophys. Res., 98, Dl 1, pp. 20663-20673.

Lovelock, J.E. & Kump, L.R. (1994). Failure of climate regulation in a geophysiological model. Nature, 369, pp. 732-734.

Marland, G., Boden, T.A., Griffin, R.C., Huang, S.F. Kanciruk, P. & Nelson, T.R. (1989). Estimates of the CO2 emissions from burning and cement manufacturing using the United Nations energy statistics and the US Bureau of Mines cement manufacturing data. ORNL/CDIAC-25. NDP-030. Oak Ridge, TN: Oak Ridge National Laboratory.

Newell, R.E., Newell, N.E., Yong Zhu & Scott, C. (1992). Tropospheric rivers? - A Pilot Study. Geophys. Res. Lett., 12, pp. 2401-2404.

Pearson, M. (1995). CO2 and stomatal functioning. TIGER eye, Newsletter of TIGER: a NERC community research programme, No 14, p. 2.

Rotmans, J. & den Elzen, M.G.J. (1993). Modelling feedback mechanisms in the carbon cycle: balancing the carbon budget. Tellus, 45B, pp. 301-320.

Roulet, N.T., Ash, R. & Moore, T.R. (1992). Low boreal wetlands as a source of atmospheric methane. J. Geophys Res.,. 97 D4, pp. 3739-3749.

Siegenthaler, U. & Sarmiento, J.L. (1993). Atmospheric carbon dioxide and the ocean. Nature, 365, pp. 119-125.

WMO Report No. 25 (1991). Scientific assessment of the ozone depletion.

TIGER Eye (1995). Newsletter of TIGER: a NERC community research programme, No. 13, p.14.